It is thought that the percentage of the friction loss that occurs between an outer periphery sliding portion of a piston ring that is a sliding member used for a reciprocating internal combustion engine, such as an automotive engine, and a bore of a cylinder liner is about 20 to 30 percent of the friction loss that occurs in the reciprocating internal combustion engine, such as an automotive engine. Reducing the friction loss may improve automotive fuel economy and reduce exhaust gas, and contribute to environmental conservation.
Conventionally, on an outer periphery surface of piston rings, for the purpose of improving wear resistance, a nitrided layer by a nitriding process, a chromium plated film, a hard film by ion plating, such as titanium nitride or chromium nitride, is formed. In addition, in recent years, coating with an amorphous hard carbon film having a high hardness and excellent self-lubrication has been used.
Amorphous hard carbon may be generally called diamond-like carbon (hereinafter referred to as “DLC”), hydrogenated amorphous carbon (a-C:H), i-carbon, or diamond-like carbon or the like, and is characterized in that it is structurally a mixture of a bonding in which the bond of carbon is a diamond structure (sp3 bonding) and a bonding in which the bond of carbon is a graphite structure (sp2 bonding), and has high hardness, wear resistance, thermal conductivity and chemical stability similar to those of a diamond, and also has solid lubrication similar to that of a graphite. Because of these characteristics, DLC has been used for a protecting layer of, for example, a sliding member for automotive components, a mold, a cutting tool, a mechanical component, or an optical component.
For example, relating to a piston ring for an automotive component, a DLC film having a surface structure on which a bulge having a size of 0.5 to 5 micrometers is deposited and is formed to a thickness of 0.5 to 30 micrometers on upper and lower surfaces of a piston ring after the entire surface of these surfaces being gas-nitrided is disclosed. This may prevent aluminum adhesion to the upper and lower surfaces of the piston ring that slides against an aluminum alloy, and reduce the ring groove wear of a piston made of aluminum alloy (see Japanese Patent Publication JP-A-2000-120869). Another example is a DLC film having a thickness of 0.5 to 30 micrometers formed on an underlying film that may be formed either directly on upper and lower surfaces of a piston ring or on a hard surface-treated layer, such as a chromium plated film, a gas-nitrided layer or the like. The underlying film is made of 70 to 100 atomic percent of one type or more than two types of elements selected from the group consisting of Si, Ti, W, Cr, Mo, Nb and V and the remaining content of the film consists of carbon. This may reduce the ring groove wear of a piston made of aluminum alloy (see Japanese Patent Publication JP-A-2000-120870).
A combination of a cylinder made of aluminum alloy and a piston ring with a DLC film being formed on an outer periphery surface thereof is also disclosed (see Japanese Patent Publication JP-A-2001-280497).
In Japanese Patent Applications JP-A-05-163909 and JP-A-07-118832, relating to a cam contact portion structure of a valve mechanism for an internal combustion engine, methods for reducing friction loss by using a hard film are disclosed.
Possible ways effective in reducing the friction loss that occurs between a piston ring outer periphery sliding portion and a cylinder liner bore may be; reducing the tension of the piston ring, optimizing the shape of the piston ring outer periphery, reducing the surface roughness of the piston ring outer periphery, or reducing friction coefficient by surface treatment of the piston ring outer periphery. Another effective way may be to reduce the surface roughness of the cylinder liner.
However, if the tension of the piston ring is excessively reduced, oil consumption may increase or the amount of blowby-gas may increase. In addition, if the surface roughness of the piston ring outer periphery is excessively reduced, seizing-up (scuffing) may occur. Furthermore, the surface roughness of the piston ring outer periphery may gradually decrease as the piston ring, according to the related arts described above, slides in the cylinder liner, and the friction loss that occurs between the piston ring outer periphery sliding portion and the cylinder liner bore may tentatively decrease; however, the wear of the piston ring outer periphery sliding surface may proceed and thus the optimally designed initial shape cannot be maintained. Consequently, the friction loss that occurs between the piston ring outer periphery sliding portion and the cylinder liner bore may increase.
In Japanese Patent Publication JP-A-2000-120869, a sliding surface of the piston ring may be treated to have a surface roughness (Ra) of 0.05 to 1 micrometers, and then the surface is coated with a DLC film having a Vickers hardness of Hv 700 to 2000 and a thickness of 0.5 to 30 micrometers. However, with the film having such a low hardness, self-wearing of the film may increase when it slides against a cylinder liner made of cast iron, and thus break-in effect due to the improvement in the surface roughness of a mating material cannot be obtained. In Japanese Patent Publication JP-A-2000-120870, it is disclosed that even when the DLC film and the underlying film are worn, the chromium plated film, the nitrided layer or an ion plated film may work for wear resistance of the piston ring; however, there is no teaching regarding the initial roughness required for breaking-in against a mating material. As described above, regarding the sliding between a sliding member, especially a piston ring, and a cast iron cylinder, most of the related arts focus on the break-in property of the piston ring, and there is no teaching regarding reduction in friction loss by the break-in effect of a cylinder material. Furthermore, the area of the sliding portion of the piston ring outer periphery and the area of the sliding portion of the cylinder liner bore differ significantly as compared with the areas of other sliding members, and the sliding conditions are different from those above mentioned Japanese Patent Applications JP-A-05-163909 and JP-A-07-118832. Therefore, in the related arts, it has been very difficult to significantly reduce the friction loss that occurs between the piston ring outer periphery sliding portion and the cylinder liner bore.